This invention relates to timing systems and more especially but not exclusively it relates to the measurement of precise time intervals between events at mutually spaced locations.
The precise timing (that is to within an accuracy of 1 nanosecond) of events, such as the period between reception of pulses, or the generation of synchronised pulses, presents little difficulty when the pulses are received or produced (as the case may be) at the same location. However, precise timing of events at locations spaced several kilometres apart, is much more problematic if line of sight communication between the sites is not possible. In order to provide the required degree of accuracy with known systems a very stable (and therefore expensive) reference clock, such as a Caesium frequency source, must be used in conjunction with a basic GPS Common View synchronisation technique.
It is an object of the present invention to provide a relatively inexpensive time interval measurement system for achieving precise timing of events at widely spaced locations.
According to the present invention a time-interval measurement system for the timing between events occurring at two spaced apart locations comprises at each location a dual frequency Global Positioning System (GPS) receiver having operatively associated with it a GPS antenna, wherein both the GPS receiver and the antenna are dual frequency (L1, L2) and the former is capable of both Coarse Acquisition (C/A) Code and carrier phase measurements, an accurate frequency reference source giving local time, a data-logger for logging the GPS data, a Time Interval Counter (TIC) means is used to measure the time of a local event relative to the local time and further comprising a central processor system (CPS) and a communication system via which the logged GPS data and time interval data are received at the CPS from each location, the CPS being arranged to derive a time-offset figure, in accordance with a predetermined algorithm which is indicative of the difference between times as measured in dependence upon the frequency reference sources at the two locations.
It will be appreciated that the time interval figure can be applied to the TIC measurements to calculate precise, relative, time difference between events occurring at the two locations.
The frequency source may be integral with the GPS receiver. Alternatively the GPS receiver may be adapted to receive a clock signal from an external source.
The frequency source may, for example, be a quartz crystal oscillator or a rubidium oscillator, which provide good short-term stability at modest cost.
The dual frequency GPS antenna may have calibrated group delay characteristics over temperature. Alternatively the antenna may comprise a passive dual-band patch with a short cable run, thereby removing the need for amplifiers which might give non-reproducible temperature characteristics.
The data logger may be a personal computer (PC). The CPS may be co-located at one of the GPS receiver locations and may also comprise a PC.
It will be appreciated that the system may be used in applications where the timing of events at several widely spaced locations is required, the appropriate time intervals being computed in each case.
The system may, for example, receive a designated pulse at two or more locations. The time interval measurements can then be applied to the local TIC measurements to allow the precise calculation of the absolute time offset between the pulses. Alternatively the system may feed back the measured time intervals to synchronise operational events at two or more locations.